/Engine/Plugins/Online/OnlineFramework/Source/Party/Public/Party/PartyMember.h
Bummer to have two signatures, but cases that want both the old and new values are much rarer, so most don't want to bother with a handler that takes an extra unused param
typedef TMulticastDelegate_OneParam< void, [CrossplayPreferenceArgType](API\Plugins\Party\Party\FPartyMemberRepData\CrossplayPreferenceArgType) > FOnCrossplayPreferenceChanged